Dear Felix
I am delighted by your offer to join [FIRM, ex. XYZ Inc]. Although I find the position and job duties to be challenging and tailored to my abilities, I have reluctantly decided not to accept the offer [STATE REASON, ex. as I would prefer to stay in the Phoenix area].
It was a pleasure to meet with you and I thank you for the time you gave me. I wish yourself and [FIRM, ex. XYZ Inc.] the best in the future.
Regards
Lydia